The present invention is directed to vacuum tube amplifiers, and more particularly, to a biasing and control circuit for varying the level of output power of vacuum tube push-pull amplifiers.
Much of today's music is performed using electric instruments and amplification systems which allow musicians to achieve various special effects not previously possible. Certain of these special effects can be produced by introducing "clean distortion" in the output stage of an amplifier. For example, by intentionally introducing clean distortion in the output stage of the amplifier of an electric guitar, a musician can make the guitar sound like a bassoon or a tenor sax. Clean distortion can be described as a clipping of the peaks of signals which are otherwise a true (often amplified) reproduction of signals previously generated by a signal source such as an electric instrument. It is to be differentiated from other forms of distortion in that it is usually intentionally introduced for the purpose of producing desired special effects. One way clean distortion can be introduced into an amplification system is to limit the amount of output power which can be delivered by the output stage of the amplifier, while simultaneously increasing the gain of the pre-amp driving the output stage until such stage becomes overdriven.
Many of the amplifiers for electric instruments manufactured today use vacuum tubes because of the "warm"sound which they produce. Such amplifiers are typically designed with a push-pull output stage to allow the amplifiers to operate at a high level of efficiency, while simultaneously being capable of producing the substantial amounts of power often demanded by musicians. Push-pull vacuum tube amplifiers are often designed with either fixed biasing or cathode biasing to deliver a particular level of output power. In fixed biasing designs, the cathodes of the tubes comprising the push-pull output stage are normally grounded, while a negative bias is applied to the control grids of the tubes. In cathode biasing designs, the cathodes of the tubes are held at a fixed potential above ground, typically by means of a fixed high wattage resistor, and no negative bias is applied to the tubes' control grids.
Certain push-pull tube amplifiers are designed to deliver more than one level of output power, however, such amplifiers typically deliver either only a high power output or a low power output. One design of this type switches the windings of its power transformer to produce either a high or low B+working voltage, and thereby, to produce the high and low power levels. Another design utilizes four power tubes and simply lifts the cathodes of two of the tubes (one from each bank) from ground to produce the high and low power levels. Although such designs allow a user the option of two different power levels, neither provides the output variability necessary for a musician to produce special effects in accordance with the size of the location in which he is playing
Variable levels of output power have been available in tube amplifiers in the past through the use of an extremely high power rheostat connected to the secondary of the amplifier's output transformer. Although the rheostat allows the output power of the amplifier to be varied, the rheostat nevertheless causes a mismatch between the impedance of the transformer's secondary winding and the impedance of the speaker loads. This mismatch causes a distortion to be introduced into the output signals of the amplifier which can not be eliminated if desired. This method of varying output power also very often causes premature failure of the output transformer and/or the power output tubes themselves.
Accordingly, it is an object of the present invention to provide an output stage for a tube amplifier which allows the output power delivered by the amplifier to be manually varied.
It is another object of the present invention to provide a biasing and control circuit for the push-pull output stage of a vacuum tube amplifier which allows the output power delivered by the output stage to be manually varied within a specified power range.
It is a further object of the present invention to provide a biasing and control circuit which allows the power output of a push-pull vacuum tube amplifier to be varied so that clean distortion can be introduced into the amplifier no matter what power output constraints are imposed on the amplifier by the size of the location in which it is used.